Unfortunately this is a known undone (see line 948 in http://orm.svn.sourceforge.net/viewvc/orm/trunk/RelationalModel/OialDcilBridge/NameGeneration.cs?revision=1459&content-type=text%2Fplain) which says // UNDONE: The PK, UC, FK suffixes should have generator options to control them.
These names are combinations of table names, constraint names (generally used only if they are not the default 'Constraint#' pattern), the suffix, and possibly a trailing number. Other inputs (such as column names) could also be used. The proper fix here is to allow format string similar to the relational name reference mode formatters that would give you broad control over use of replacement fields.
I don't think I'll get to this soon, but I'm attaching a stopgap that modifies the decorated constraint names at the XML level. Follow directions in the readme for installation and use. This is similar to the answer I gave you for http://www.ormfoundation.org/forums/p/581/1738.aspx#1738 (except I'm not leaving the transform as an exercise).